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I have a flame rollout in the #2 combustion chamber that begins at 2-3 minutes after the main blower starts.I was having a flame rollout in both #2 and #3 that began immediately after ignition. I had the unit professionally cleaned/inspected, which eliminated the #3 problem. The tech found no cracks in the exchanger. The flue is clear. There is venting at the #2 exhaust. The combustion fan is running and appears strong. We even tried removing the baffle at the top of the exchanger. Still, the tech was unable to diagnose the problem and recommended replacement. Any suggestions?

The GUA is forced draft. You have to have the burner box cover securly on when running the furnace. If not, you will have rollout. Still, seems funny it is only 1 cell. Could be a crack the tech isn't finding.

does that model have a sealed burner cover? if so tape off the intake and exhaust pipes..... hook your magnehelic up to the burner box pressure port....cycle fan off and on with the switch....if you get a reading you have a hole in a heat exchanger....

Does this unit have the outside combustion air intake connected to the draft motor? If it does, disconnect it. These units had a tendency to get rusty burners due to humid air from the intake. Rusty burners=incomplete combustion=sooty heat exchanger=flame roll-out.

The unit was overfiring. The manifold pressure was at 4.0", and btu/hour was 96k (by meter check). Nameplate is 3.5" and 80k btu/hour. I eliminated the rollout problem by reducing the pressure to 3.4, which resulted in a btu/hour of 84k. My question now is, which is more important, manifold pressure, or btu/hr?